DESCRIPTION OF A NEW SPECIMEN OF PLESIOSAUR AND IMPLICATIONS OF THE BIODIVERSITY OF THE TROPIC SHALE OF SOUTHERN UTAH

Recent fieldwork conducted by the Museum of Northern Arizona has resulted in the recovery of more than a dozen short-neck plesiosaur skeletons from the marine Tropic Shale (Cenomanian-Turonian) in Glen Canyon National Recreation Area and Grand Staircase-Escalante National Monument of southern Utah. To date, two new species of plesiosaur, within two new subfamilies have been recognized in the Tropic Shale fauna. Two previously recognized species have also been discovered in the Tropic Shale. The skeleton of another plesiosaur was excavated in 2005 from a site in Glen Canyon National Recreation Area and is the first nearly complete individual recovered during the past six years of intensive fieldwork in this area. The skeleton is remarkably well preserved, although heavily fractured. The specimen consists of the skull, an incomplete set of vertebrae and ribs, the majority of the pectoral girdle, nearly complete forelimbs, the majority of the pelvic girdle, nearly complete hindlimbs, and associated paleogastroliths.

Following preparation and description, the new plesiosaur skeleton can be placed within the Family Polycotylidae. Several characters are similar to those seen in other polycotylid plesiosaurs. However, the unique combination of characters found in the new specimen, as well as the presence of several previously unrecognized characters permits the assignment of a new species. Additionally, several characters are similar to one genus of polycotylid (Dolichorhynchops) while several other characters are similar to another genus of polycotylid (Trinacromerum). This suggests that the new specimen could be an intermediate between the two genera, permitting the synonomy of these two polycotylids.

All of the specimens described from the Tropic Shale in the past six years, including the new specimen described indicate a high diversity of marine predators present during the Cenomanian and Turonian along the western border of the Cretaceous Western Interior Seaway.